25.4.2 Standard Environment Variables

These environment variables have standard meanings. This doesn’t mean
that they are always present in the environment; but if these variables
are present, they have these meanings. You shouldn’t try to use
these environment variable names for some other purpose.

HOME

This is a string representing the user’s home directory, or
initial default working directory.

The user can set HOME to any value.
If you need to make sure to obtain the proper home directory
for a particular user, you should not use HOME; instead,
look up the user’s name in the user database (see User Database).

For most purposes, it is better to use HOME, precisely because
this lets the user specify the value.

LOGNAME

This is the name that the user used to log in. Since the value in the
environment can be tweaked arbitrarily, this is not a reliable way to
identify the user who is running a program; a function like
getlogin (see Who Logged In) is better for that purpose.

For most purposes, it is better to use LOGNAME, precisely because
this lets the user specify the value.

PATH

A path is a sequence of directory names which is used for
searching for a file. The variable PATH holds a path used
for searching for programs to be run.

The execlp and execvp functions (see Executing a File)
use this environment variable, as do many shells and other utilities
which are implemented in terms of those functions.

The syntax of a path is a sequence of directory names separated by
colons. An empty string instead of a directory name stands for the
current directory (see Working Directory).

A typical value for this environment variable might be a string like:

:/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin

This means that if the user tries to execute a program named foo,
the system will look for files named foo, /bin/foo,
/etc/foo, and so on. The first of these files that exists is
the one that is executed.

TERM

This specifies the kind of terminal that is receiving program output.
Some programs can make use of this information to take advantage of
special escape sequences or terminal modes supported by particular kinds
of terminals. Many programs which use the termcap library
(see Find in The Termcap Library
Manual) use the TERM environment variable, for example.

TZ

This specifies the time zone. See TZ Variable, for information about
the format of this string and how it is used.

LANG

This specifies the default locale to use for attribute categories where
neither LC_ALL nor the specific environment variable for that
category is set. See Locales, for more information about
locales.

LC_ALL

If this environment variable is set it overrides the selection for all
the locales done using the other LC_* environment variables. The
value of the other LC_* environment variables is simply ignored
in this case.

LC_COLLATE

This specifies what locale to use for string sorting.

LC_CTYPE

This specifies what locale to use for character sets and character
classification.

LC_MESSAGES

This specifies what locale to use for printing messages and to parse
responses.

LC_MONETARY

This specifies what locale to use for formatting monetary values.

LC_NUMERIC

This specifies what locale to use for formatting numbers.

LC_TIME

This specifies what locale to use for formatting date/time values.

NLSPATH

This specifies the directories in which the catopen function
looks for message translation catalogs.

_POSIX_OPTION_ORDER

If this environment variable is defined, it suppresses the usual
reordering of command line arguments by getopt and
argp_parse. See Argument Syntax.